Power supply system

ABSTRACT

An electrical power supply system for efficiently supplying power at multiple voltage levels to various loads. The system includes transformer means intercoupling a plurality of different voltage levels so as to automatically translate or redistribute power as load conditions change. For example, as load current directions change, power is translated with minimum loss from levels acting as a current sink to levels requiring a current source.

United States Patent MXX W( 2 5 M 7 33 2,811,688 10/1957 Kittl 2,983,8605/1961 Todd........ 3,167,685

1/1965 Badeetal..... .......1....:..

Aug. 11, 1969 Patented Aug. 17, 1971 m l 8 C m 0 mm: 3 m v.09 0.."4 RT8r o m N a a v P h A l 2 .l 7 2 l l [22] Filed FOREIGN PATENTS 1/1947GreatBritain...............,

Primary ExaminerRobert K. Schaefer Assistant Examiner-William J. SmithArtorney Frederick M. Arbuckle ABSTRACT: An electrical power supplysystem for efficiently supplying power at multiple voltage levels tovarious loads.

1 8 w m1 1 m 7 3 MN [50] Field of 44, 74, 75, 80, 85, 87; The systemincludes transformer means intercoupling a plu- I 321/49 rality ofdifferent voltagelevels so as to automatically translate or redistributepower as load conditions change. For example,

as load current directions change, power is translated with [56References Cited UNITED STATES PATENTS PATENIED'Aur; I 7 I97! SHEET 3[IF 3 FIG. 3

INVENTOR. ROY P. FOERSTER BY 1M, W

' ATTORNEYS POWER SUPPLY SYSTEM BACKGROUND OF THE INVENTION 'l. Field ofthe Invention The present invention relates to improvements in powersupply systems particularly useful in electronic systems such as digitalcomputer systems for supplying power thereto at multiple voltage levels.

Many electronic systems, and particularly digital computer systems,incorporate multiple voltage levels for such purposes as clamping logiclevels, decision reference levels and bias supplies. The use of multiplevoltage levels is desirable inasmuch as it permits the design of a morerigorous system and generally improves circuit flexibility andreliability. There are, however, a number of problems arising from theneed for supplying these multiple voltage levels. Some of the problemsinvolve: (l) the sheer bulk of the power supply hardware required, (2)the number of adjustments which must be made in the power supplies, and(3) the complexity of design analysis needed to accommodate multiple andindependent power supply variations.

An additional problem arises due to the fact that conventional powersupplies conduct current unilaterally while power supply loads intypical electronic systems vary widely and may even become negative.That is, a particular voltage level may have to act as a current sourceone moment and as a current sink the next moment.

Description of the Prior Art The prior art in power supply design cannotrealistically achieve a supply which will conduct current bilaterally.Thus, the conventional solution to this problem has been to design thepower supply with a shunt preload so that it is never necessary for areverse current to flow within the power supply itself. In this way, thepower supply only sees a unilateral load which it can handle in a normalmanner. It will be readily recognized, however, that this techniqueresults in a considerable amount of lost power in the shunt path.

SUMMARY OF THE INVENTION powerefficiencies. I

Briefly, in accordance with the present invention, a power supply systemis provided which uses a plurality of pairs of bilateral synchronousswitches in conjunction with a transformer to supply multiple DCpotential levels as defined by the transformer turns ratios. Thebilateral switch pairs permit the flow of power to the various levels tobe automatically balanced with very low loss.

In a preferred embodiment of the invention, a single main DC powersupply is connected through a pair of switches to the terminals of anautotransformer. The switches are alternately driven on and off by asquare wave oscillator to convert the supplied DC potential to AC. Otherswitch pairs driven by the same oscillator are connected toappropriately located taps along the autotransformer to convert AC backto DC at the desired voltage levels.

In accordance with a significant aspect of the invention the switchpairs are operated bilaterally to thus permit power to flow from atransformer tap to a potential level supply bus or from a bus through atap for redistribution by the transformer to other voltage levels.

A system in accordance with the invention can be very simply andreliably implemented. By using only low loss elements, e.g., transistorswitches, very high power efficiencies are readily realized. Noadjustments are required within the power supply system and outputvoltage levels are inherently fixed by the level of the main powersupply and the turns ratios of the transformer. This arrangement assuresthat voltage variations occur proportionately, rather thanindependently, as is the case in conventional prior art systems. Thenumber of voltage levels which can be provided is limited only by thenumber of taps on the transformer and the number of transistor switchpairs utilized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1(a) is a block diagram of atypical prior art power supply system;

FIG. 1(b) is a schematic diagram of a typical digital computer circuitshown for the purpose of demonstrating how loads can sometimes act assinks and at other times act as sources with respect to particularpotential levels in a multilevel system;

FIG. 2 is a block diagram of a power supply system embodying the presentinvention; and

FIG. 3 is a more detailed block diagram of the system of FIG. 2.

Attention is now called to FIG. 1(a) which illustrates a typical priorart power supply system for supplying DC power at multiple voltagelevels to an electronic system such as a digital computer system. Theelectronic system is normally comprised of many different loads intendedto operate between various potential levels. Thus, for example, theexemplary system of FIG. 1(a) includes loads L1, L2, L3 and L4 intendedtooperate respectively between +18 volts, +7 volts, +4 volts, -12 volts,and ground. Additionally, a load L5 is intended to operate between +18volts and +7 volts, a load L6 between +7 volts and +4 volts, and a loadL7 between +18 volts and +4 volts.

In order to apply the indicated voltage levels across the various loads,it is common practice to utilize a plurality of separate power supplymodules, each providing DC power at a different voltage level. Forexample, power supply module PS1 can provide DC power at +18 volts.Similarly, power supply modules PS2, PS3, and PS4 can respectivelyprovide DC power at +7 volts, +4 volts, and l2 volts. Thus, as is shownin FIG. 1(a), loads L1, L2, L3, and L4 will be respectively connectedbetween ground and the output terminals of power supply modules PS1,PS2, PS3, and PS4. Load L5 is connected between the output terminals ofpower supply modules PS1 and PS2, load L6 between the output tenninalsof power supply modules PS2 and PS3, and load L7 between the outputterminals of power supply modules PS1 and PS3.

Typical power supply modules constitute unilateral current conductingdevices and are schematically illustrated in FIG. 1(a) as including adiode. Thus, each of the power supply modules of FIG. 1(a) can act as acurrent source providing DC power at a specific voltage level. However,in typical electronic systems, it is sometimes necessary for the variousvoltage levels to function at one moment as a current source and atanother moment as a current sink;

More particularly, as is shown in FIG. 1(a) power supply module PS1provides an output current comprised of load current components I1, l5,and I7 respectively flowing through loads L1, L5, and L7. Power supplymodule PS2 supplies load currents I2 and I6 through loads L2 and L6respectively. As load conditions change, it may occur that the loadcurrent component I5 exceeds the magnitude of the sum of the loadcurrent components I2 and I6. Since the power supply module PS2 is aunilateral current conducting device, some means must be provided forhandling the excess current supplied by component l5. In typical priorart systems, this excess current is handled by providing a shuntresistor R,2 connected between the output of power supply module PS2 andground. That is, when the +7 volt level is acting as a current sink,resistor R, 2 passes the current I,2=I5(l2+l6).

Similarly, the +4 volt level will act as a current sink when the sum ofthe current components I6 and I7 exceeds current component I3. In orderto handle this excess current, a resistor R3 is connected between theoutput of power supply module PS3 and ground. Resistor R,3 will draw thecurrent I 3=(I6 +17) I3.

In order to better understand why certain ones of the various voltagelevels in the system of FIG. 1(a) can at different times operate aseither current sources or current sinks, attention is now called to FIG.1(b) which illustrates a circuit typical of the type employed in digitalcomputer systems. Briefly, the circuit of FIG. 1(b) comprises an ANDgate controlling a transistor switch Q1. When the potentials applied tothe input terminals A and B are both high, e.g., +18 volts, then theoutput potential will be at ground. On the other hand, when groundpotential is applied to either input terminal A or B, then the outputpotential will be at +7 volts.

More particularly, the circuit of FIG. 1(5) includes an NPN switchingtransistor Q1 whose collector is connected through a resistor R1 to the+18 volt level. Additionally, the collector of transistor Q1 isconnected through clamping diode D1 to the +7 volt level. The emitter oftransistor Q1 is grounded. The base of transistor Q1 is connectedthrough a resistor R2 to the -l 2 volt level and through resistor R4 tothe output and an AND gate comprised of diode D2 and D3 and resistor R3.The

other terminals of diodes D2 and D3 are respectively connected to inputterminals A and B and the other terminal of resistor R3 is connected tothe +18 volt level.

In the operation of the circuit of FIG. 1(b), assume binary and l levelsat input terminals A and B are respectively represented by ground and+18 volt potentials. If a binary 0" is applied to either input terminalA or B, the AND gate output terminal will be at ground. On the otherhand, if binary ls" are applied to both input terminals A and B, thenthe AND gate output terminal 10 will be at some positive potential levelsufficient to forward bias transistor O1 to thus draw an increasedcurrent from the +18 volt level through resistor R1. In the absence ofthe clamping diode DI the collector of transistor Q1 would be at about+18 volts when transistor Q1 is not conducting and at about groundpotential when transistor Q1 is conducting. However, as a consequence ofthe diode D1, when transistor Q1 is not conducting, its collector is atabout +7 volts and when transistor Q1 conducts, its collector falls toabout ground potential.

Thus, from the foregoing explanation of the operation of the circuit ofFIG. 1(b), it will be recognized that clamping diode D1 will drawcurrent from +l8volt level only when transistor Q1 is not conducting. ifthe current through diode D1 is considered the component in FIG. 1(a),then it will be recognized that in a complex digital computer systemcomprised of a multiplicity of circuits of the type generally shown inFIG. 1(b), many of the voltage levels can at different times act ascurrent sources or current sinks. As previously pointed out inconjunction with FIG. 1(a), in order to tolerate this conditionutilizing unilaterally conducting power supply modules, it is necessaryto provide shunt resistor, as R,2 and R,3, so that the power supplymodules will at all times see a unidirectional load. However, it will berecognized that the current drawn through the shunt resistors representswasted power. The present invention is directed to a system whichautomatically redistributes power between the various voltage levels asneeded. That is, as load current directions change, power from a levelacting as a current sink is translated, with minimum loss, to a levelrequiring a current source.

Attention is now called to F IG. 2 of the drawing which illustrates ablock diagram of a power system embodying the present invention. Thesystem of FIG. 2 employs the same power supply modules as in the systemof FIG. 1(a). That is, the system of FIG. 2 includes a +18 volts, +7volt, +4 volt, and -l2 volt power supply, respectively identified asPS1, PS2, PS3, and PS4. As previously pointed out, the power supplymodules can be considered as unilaterally current conducting modules, asrepresented by the diodes contained within the power supply moduleblocks. The same load devices as are shown in the system of FIG. 1(a)are also shown in the system of FIG. 2. Thus, loads L1, L2',L3 and L4are respectively connected between the output terminals or supply bussesof the power supply modules PS1, PS2, PS3, PS4 and ground. Loads L5, L6,and L7 are connected similarly to the corresponding loads shown in FIG.I( a).

In accordance with the present invention, a transformer 20, preferablyan autotransformer, is provided for redistributing power between thevarious voltage levels so that as load current directions change, powerfrom a level acting as a current sink can be translated, with minimumloss, to a level requiring a current source.

The autotransformer is comprised of a single coil 22 having first andsecond terminals 24 and 26 and a center tap 27 connected to ground. Asingle pole double throw switch 28 couples the supply bus of powersupply module PS1 to the terminals 24 and 26. The switch 28 is driven byswitch drive means 30 between the terminals 24 and 26 to thereby apply asquare wave across the winding 22.

Each of the other power supply modules PS2, PS3, and PS4 is connectedthrough single pole double throw switches 32, 34, and 36 respectively topairs of taps appropriately positioned along the winding 22. Moreparticularly, the pole of switch 32 cooperates with taps 38 and 40, thepole of switch 34 with taps 42 and 44 and the pole of switch 36 withtaps 46 and 48. All of the switches 28, 32 34 and 36 are synchronouslydriven by the drive means 30. Thus, as +18 volt power supply module PS1applies a square wave across the winding 22, each of the synchronouslydriven poles of switches 32 34 and 36 picks up a DC potential defined bythe transformer turns ratio for application to the supply bus of thepower supply module to which it is connected. In the event the load L5,for example, supplies more current than is drawn by loads L2 and L6, theexcess current, instead of being shunted through a resister, is appliedthrough the switch 32 to the transformer winding 22 for redistributionto one of the other levels requir ing a current source. i t

It will be noted that in order to derive the -l2 volt leve from thetransformer winding 22, the taps 46 and 48 associated with switch 36 aremerely reversed with respect to the center tap 27 of the winding 22.

Whereas the system shown in FIG. 2 has been illustrated as utilizingelectromechanical single pole double throw switches, it will berecognized that in an actual embodiment of the invention, it would bepreferable to utilize electronic switches such as transistors. Apractical implementation of an embodiment of the invention isillustrated in FIG. 3 wherein the primed elements of FIG. 2 are doubleprimed.

More particularly, the embodiment of FIG. 3 includes power supplymodules PS1", PS2", PS3", and PS4" respectively providing DC power atvoltage levels of +18 volts, +7 volts, +4 volts, and l2 volts. Loads L1L7" are connected to the power supply modules in the same manner as thecorresponding loads in FIG. 2. In the embodiment of FIG. 3 each of thebilaterally conducting switches 28", 32", 34" and 36" is comprised of apair of PNP transistors Q2 and Q3. The emitters of transistors Q2 and Q3are connected in common and to the supply bus of the corresponding powersupply module. Thus, the output of power supply module PS1" is connectedto the emitters of transistors Q2 and Q3 of switch 28'. The collectorsof the transistors are connected to terminals 24" and 26" of atransformer winding 22". The emitters of transistors Q2 and Q3 of aswitch 28" are connected through a resistor 50 to the center tap of atransformer secondary winding 52. The terminals of winding 52 areconnected across the bases of transistors Q2 and Q3 of switch 28". Theswitches 32", 34" and 36" are implemented identically to the switch 28The transistors Q2 and Q3 of each of the switches are alternatelyenergized by an oscillator 54 (corresponding in function to the switchdrive means 30 of FIG. 2) comprised of transistors Q4 and Q5 and drivenby the output of the +18 volt power supply module PS1 The collectors oftransistors Q4 and Q5 are coupled to each other through a transformerwinding 56 inductively coupled to the plurality of secondary windings 52each synchronously driving the different one of the switches 28", 32",34" and 36". The oscillator 54 construction shown in FIG. 3 isconventional and is only exemplary of several different circuitarrangements which can be employed.

The embodiment of FIG. 3 operates identically to the schematicallyillustrated embodiment of FIG. 2 in that each of the transistors Q2 andQ3 of each of .the switches can bilaterally conduct current to and fromthe autotransformer winding 22' to thus enable the winding 22' toautomatically redistribute power between the various voltage levels.

From the foregoing, it should be recognized that a particularlyefficient power supply system has been disclosed herein for providing DCpower to a plurality of loads at multiple voltage levels. Powerefficiency is achieved by utilizing a trans- .former for automaticallyredistributing power between various voltage levels through bilateralswitches, as load current conditions change.

1 claim:

l. A power supply system for supplying DC power at different voltagelevels to multiple supply busses for application to independentlyoperated loads connected between various busses and wherein at leastsome of said busses can at various times act either as a current sink ora current source, said system comprising:

a transformer having first and second terminals and at least two tappairs each comprised of spaced first and second taps; a main powersupply module capable of providing DC power at a first potential level;

second and third power supply modules respectively capable of providingDC power at second and third potential levels each different from saidfirst level;

a first switching means for selectively defining first and second statesrespectively coupling said main power supply across said first andsecond terminals in first and second polarity directions;

a second switching means including first and second bidirectionalcurrent conducting switches respectively connected between first andsecond taps of one of said tap pairs and a common second switching meansoutput terminal;

means for connecting said second power supply module to said secondswitching means output terminal;

a third switching means including first and second bidirectional currentconducting switches respectively connected between first and second tapsof a second of said tap pairs and a common third switching means outputterminal;

means for connecting said third power supply module to said thirdswitching means output terminal;

said tap pairs being located in accordance with the potential levels ofthe power supply modules to which they are to be connected; and

means for alternately switching said first switching means between saidfirst and second states and for synchronously closing said second andthird switching means first switches coincident with said first stateand said second and third switching means second switches coincidentwith said second state so as to convert the AC signals produced at saidtap terminals into DC levels appropriate for said second and third powersupply modules respectively connected thereto.

2. The system of claim 1 wherein said transformer means comprises anautotransformer.

3. The system of claim 1 wherein each of said second switching meanscurrent conducting switches comprises a semiconductor having a controlterminal and first and second current conducting terminals; and

means connecting said first current conducting terminals of said secondswitching means current conducting switches to said first and secondtaps of said one tap pair and said second current conducting terminalsof said second switching means current conducting switches to saidsecond switching means out ut terminal. 4. The system of claim 3 incuding oscillator means; and

means coupling said oscillator means to said control terminals of saidsecond switching means current conducting switches.

5. In an electronic system including a plurality of power supply moduleseach providing DC power at a different voltage level to a plurality ofload devices each connected between a pair of voltage levels, a powerdistribution system for distributing power from a level which wouldotherwise act as a current sink to a level acting as current source,said distribution system including:

a transformer means including a pair of terminals and a plurality ofpairs of taps located in accordance with the signal levels to beprovided thereat;

a first switching means coupling one of said power supply modules acrosssaid pair of terminals;

a plurality of additional switching means each coupling a different oneof said power supply modules across a different pair of said taps, saidpairs of taps being located in accordance with the potential levels ofthe power supply modules to which they are to be connected;

each of said switching means including first and second bidirectionalcurrent conducting switches, each connected between a power supplymodule and different taps or terminals of a common pair; and

means for alternately switching said first andsecond current conductingswitches of all of said switching means in synchronism so as to convertthe AC signals produced at said tap terminals into DC levels appropriatefor said second and third power supply modules respectively connectedthereto.

6. The system of claim 5 wherein each of said bidirectional currentconducting switches comprises a semiconductor having a control terminaland first and second current conducting terminals; and

means connecting the first current conducting terminals in eachswitching means in common and the second current conducting terminals ineach switching means to different taps or terminals of a common pair.

7. The system of claim 6 including oscillator means; and

means coupling said oscillator means to the control terminals of all ofsaid switching means.

1. A power supply system for supplying DC power at different voltagelevels to multiple supply busses for application to independentlyoperated loads connected between various busses and wherein at leastsomE of said busses can at various times act either as a current sink ora current source, said system comprising: a transformer having first andsecond terminals and at least two tap pairs each comprised of spacedfirst and second taps; a main power supply module capable of providingDC power at a first potential level; second and third power supplymodules respectively capable of providing DC power at second and thirdpotential levels each different from said first level; a first switchingmeans for selectively defining first and second states respectivelycoupling said main power supply across said first and second terminalsin first and second polarity directions; a second switching meansincluding first and second bidirectional current conducting switchesrespectively connected between first and second taps of one of said tappairs and a common second switching means output terminal; means forconnecting said second power supply module to said second switchingmeans output terminal; a third switching means including first andsecond bidirectional current conducting switches respectively connectedbetween first and second taps of a second of said tap pairs and a commonthird switching means output terminal; means for connecting said thirdpower supply module to said third switching means output terminal; saidtap pairs being located in accordance with the potential levels of thepower supply modules to which they are to be connected; and means foralternately switching said first switching means between said first andsecond states and for synchronously closing said second and thirdswitching means first switches coincident with said first state and saidsecond and third switching means second switches coincident with saidsecond state so as to convert the AC signals produced at said tapterminals into DC levels appropriate for said second and third powersupply modules respectively connected thereto.
 2. The system of claim 1wherein said transformer means comprises an autotransformer.
 3. Thesystem of claim 1 wherein each of said second switching means currentconducting switches comprises a semiconductor having a control terminaland first and second current conducting terminals; and means connectingsaid first current conducting terminals of said second switching meanscurrent conducting switches to said first and second taps of said onetap pair and said second current conducting terminals of said secondswitching means current conducting switches to said second switchingmeans output terminal.
 4. The system of claim 3 including oscillatormeans; and means coupling said oscillator means to said controlterminals of said second switching means current conducting switches. 5.In an electronic system including a plurality of power supply moduleseach providing DC power at a different voltage level to a plurality ofload devices each connected between a pair of voltage levels, a powerdistribution system for distributing power from a level which wouldotherwise act as a current sink to a level acting as current source,said distribution system including: a transformer means including a pairof terminals and a plurality of pairs of taps located in accordance withthe signal levels to be provided thereat; a first switching meanscoupling one of said power supply modules across said pair of terminals;a plurality of additional switching means each coupling a different oneof said power supply modules across a different pair of said taps, saidpairs of taps being located in accordance with the potential levels ofthe power supply modules to which they are to be connected; each of saidswitching means including first and second bidirectional currentconducting switches, each connected between a power supply module anddifferent taps or terminals of a common pair; and means for alternatelyswitching said first and second current conducting switches of all ofsaid switching means in syncHronism so as to convert the AC signalsproduced at said tap terminals into DC levels appropriate for saidsecond and third power supply modules respectively connected thereto. 6.The system of claim 5 wherein each of said bidirectional currentconducting switches comprises a semiconductor having a control terminaland first and second current conducting terminals; and means connectingthe first current conducting terminals in each switching means in commonand the second current conducting terminals in each switching means todifferent taps or terminals of a common pair.
 7. The system of claim 6including oscillator means; and means coupling said oscillator means tothe control terminals of all of said switching means.